window: Keep windows with placement rule attached to parent

A window placed using a placement rule should keep that relative position even if the parent window moves, as the position tied to the parent window, not to the stage. Thus, if the parent window moves, the child window should move with it.

In the implementation in this commit, the constraints engine is not used when repositioning the children; the window is simply positioned according to the effective placement rule that was derived from the initial constraining, as the a xdg_popup at the moment cannot move (relative its parent) after being mapped.

Closes: #274 (closed)